1. Field of the Invention
The present invention relates to a chromate surface treated steel sheet having very excellent paintability, corrosion resistance, and workability, more specifically surface treated steel sheets for paint coating, having very excellent properties, specifically with respect to corrosion resistance after paint coating, paint adhesion in services long after the paint coating (so-called secondary paint adhesion), corrosion resistance of paint coating defect portions, and resistance to secondary work cracking.
2. Description of the Related Arts
Conventionally, as for steel sheets which are used in the paint coated form for various containers, a surface treated steel sheet prepared by applying a dual layer coating consisting of a metallic chromium layer and a chromium oxide layer mainly composed of hydrated oxide on the surface of cold rolled steel sheet (known as TFS-CT sheet) as disclosed in Japanese Laid-Open Patent Application No. Sho 54-95941 has been widely used and has been found to give satisfactory results with respect to paint adhesion, corrosion resistance after paint coating as well as economical advantages.
The TFS-CT treated steel sheets which are prepared by the TFS-CT treatment of the steel substrate of ordinary steel composition have been widely used as steel materials for many applications, including welded cans notably for their excellent paintability, can lids or crowns for their good paint adhesion after press working and good corrosion resistance after paint coating, and paint coating steel substrates as building materials.
In recent years, increasing demands have been made for surface-treated steel sheets having various improved properties, such as improved performances to meet with the expansion and diversification of the application fields of steel sheet materials, excellent appearance quality to meet with the high quality preference tendency of consumers, and good resistance against rust formation during storage.
For example, in order to meet with the increased demand for deformed cans, such as necked-in cans, substantial improvements in corrosion resistance with respect to after-paint coating in severely worked portions of articles, improvements in paint adhesion after a long period of storage and so on have been sought for.
Also in connection with steel sheet materials for can lids, the easiness in opening of the lids have been demanded more than conventionally, and in order to meet with this demand, it is necessary to improve the corrosion resistance after paint coating after working as the thickness of the can lid material sheet is made thinner, or the score residual of score worked portion is made thinner.
Further in applications, such as can bottom lids where clear lacquer coating is applied prior to use, a beautiful whitish appearance with metallic luster is more demanded than the after-paint-coating corrosion resistance and the paint adhesion. In the field of crowns, various properties, particularly resistance against the red rust formation initiated at cut edges during storage are required to be improved in connection with sheet materials for crowns and resistance against the red rust formation caused during storage of severely worked materials for can bodies and can lids is required to be improved.
For applications other than the can containers, such as house building, particularly roofs and walls, and automobiles, demands have been increasing for steel sheet substrates for paint coating, having a long term corrosion resistance and excellent paintability as well as good press formability.
Various extensive studies and investigations have been made by the present inventors to meet with the above demands in various fields of applications, and found that all chromate treated steel sheets, particularly TFS-CT treated steel sheets, which have been widely used as paint coating substrates, are prepared from Al-killed steel containing no corrosion resistance improving element and these steel sheets have problems that they show a lower electrode potential than the paint coating applied thereon so that when damaged through the paint coating to the substrate during the working after the paint coating, rust is formed at these damaged portions to dissolve the iron, causing deterioration of the after-paint-coating corrosion resistance and the paint adhesion.
Further, at these coating damaged portions or coating defective portions, pitting corrosion is caused to deteriorate the corrosion resistance.
Still further, it has been found that when paint coated articles are exposed to a corrosive media for a long period, the edge surfaces will suffer red rust formation because the edge surfaces have a lower electrode potential than the paint coating to dissolve the iron therefrom.
Meanwhile, the demand for improved appearance of surface treated steel sheets has been arising from the fact that the steel substrates are rendered to be blackish on their surfaces due to the etching effect of the chromate bath into which the substrates are immersed to give them a chromate coating, such as TFS-CT.
For the purpose of improving the workability, steel substrated containing less carbon have been conventionally used, but these substrates are very susceptible to the secondary work cracking due to the embrittled grain boundaries caused by adverse effects of unavoidable impurities, such as P and S in the steel.